Abstract An “apparatus” operator approach is presented for the extension of the density functional theory of Hohenberg, Kohn, and Mermin. Using the Mermin entropy principle, a one‐to‐one correspondence is established between the density matrix for the system and the electron charge density for a finite‐temperature system in the presence of an apparatus. In the zero‐temperature limit in the absence of an apparatus, the Hohenberg–Kohn theory is recovered. The central aspect of this new density functional theory is that the principle of maximum entropy is applied to the system plus its surroundings under the additional constraint that the electron charge density is given. The system is treated as a subsystem of a composite system and is not necessarily in the equilibrium state as in the Mermin theory. As an example, it is shown how, in principle, excited states are encompassed by the theory.
Some aspects of the correlations between interacting electrons in a magnetic field are studied by a quantum kinetic approach, taking into account the quantization of the orbital motion of the electrons. The treatment is based on the introduction of an hierarchy of equations of motion for the
s-body density matrices and employing a decoupling scheme, valid in the self-consistent field approximation. In this way the dielectric function is introduced "naturally," and the response of the system to an applied field is calculated. The dielectric function reflects the two main effects of the correlations, i.e., the collective excitations of the system and the dynamical shielding. The dielectric function plays a role also in the "dressing" of the interacting electrons. This is shown by calculating the spectrum of the density fluctuations and the equation which governs the approach to equilibrium of the system. The method is not limited to systems in thermal equilibrium, but also applies to systems in other stationary equilibrium states.
The N-representability conditions on the reduced second-order reduced density matrix (2-RDM), impose restrictions not only in the context of reduced density matrix theory (RDMT), but also on functionals advanced in one-matrix theory such as natural orbital functional theory (NOFT), and on functionals depending on the one-electron density such as those of density functional theory (DFT). We review some aspects of the applications of these N-representability conditions in these theories and present some conclusions.